Structures and stabilities of glycine and water complexes

Interaction of Cysteine with Li+ and LiF in the Presence of (H2O) n (n = 0-6) Clusters

The interaction between cysteine with Li⁺ and LiF in the microcosmic water environment was investigated to elucidate how ions interact with amino acids and the cation-anion correlation effect involved. The structures of Cys·Li⁺(H₂O) _n and Cys·LiF(H₂O) _n (n = 0-6) were characterized using ab initio calculations. Our studies show that the water preferentially interacts with Li⁺/LiF. In Cys·Li⁺(H₂O)_0-6, Li⁺ interacts with amino nitrogen, carbonyl oxygen, and hydrophobic sulfur of Cys to form a tridentate mode, whereas in Cys·LiF(H₂O) _n , Li⁺ and F^- work in cooperation and interact with carbonyl oxygen and hydroxyl hydrogen of Cys to form a bidentate type. The neutral and zwitterionic forms are essentially isoenergetic when the water number reaches three in the presence of Li⁺, whereas this occurs at four water molecules in the presence of LiF. Further research revealed that the interaction between Li⁺/LiF and Cys was mainly electrostatic, followed by dispersion, and the weakest interaction occurs at the transition from the neutral form to zwitterionic form. Natural population analysis charge analyses show that for Cys·Li⁺(H₂O) _n , the positive charge is mostly concentrated on Li⁺ except for the system containing three water molecules. For Cys·LiF(H₂O) _n , the positive charge is centered on the LiF unit in the range n = 0-6, and at n = 5, electron transfer from Cys to water occurs. Our study shows that the contribution of anions in zwitterionic state stabilization should be addressed more generally along with cations.

Monomers of Glycine and Serine Have a Limited Ability to Hydrate in the Atmosphere

The role of atmospheric aerosols on climate change is one of the biggest uncertainties in most global climate models. Organic aerosols have been identified as potential cloud condensation nuclei (CCN), and amino acids are organic molecules that could serve as CCN. Amino acids make up a significant portion of the total organic material in the atmosphere, and herein we present a systematic study of hydration for two of the most common atmospheric amino acids, glycine and serine. We compute DLPNO/CCSD(T)//M08-HX/MG3S thermodynamic properties and atmospheric concentrations of Gly(H₂O)_n and Ser(H₂O)_n, where n = 1-5. We predict that serine-water clusters have higher concentrations at n = 1 and 5, while glycine-water clusters have higher concentrations at n = 2-4. However, both glycine and serine are inferred to exist primarily in their nonhydrated monomer forms in the absence of other species such as sulfuric acid.

Unveiling Zwitterionization of Glycine in the Microhydration Limit

Charge separation under solvation stress conditions is a fundamental process that comes in many forms in doped water clusters. Yet, the mechanism of intramolecular charge separation, where constraints due to the molecular structure might be intricately tied to restricted solvation structures, remains largely unexplored. Microhydrated amino acids are such paradigmatic molecules. Ab initio simulations are carried out at 300 K in the frameworks of metadynamics sampling and thermodynamic integration to map the thermal mechanisms of zwitterionization using Gly(H₂O) _n with n = 4 and 10. In both cases, a similar water-mediated proton transfer chain mechanism is observed; yet, detailed analyses of thermodynamics and kinetics demonstrate that the charge-separated zwitterion is the preferred species only for n = 10 mainly due to kinetic stabilization. Structural analyses disclose that bifurcated H-bonded water bridges, connecting the cationic and anionic sites in the fluctuating microhydration network at room temperature, are enhanced in the transition-state ensemble exclusively for n = 10 and become overwhelmingly abundant in the stable zwitterion. The findings offer potential insights into charge separation under solvation stress conditions beyond the present example.